1. PHYS 1441 – Section 002 Lecture #10
Wednesday, Mar. 4, 2009
Dr. Jaehoon Yu
Exam Solutions
Application of Newton’s Laws
Motion without friction
Forces of Friction
Motion with friction
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

2. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Announcements
Changes of Exam Dates
Mid-term exam scheduled on Mar. 11 now moved to Wed. Mar. 25
Second non-comprehensive exam scheduled on Apr. 13 now moved to Wed. Apr. 22
Final comprehensive exam stays on the same date, May 11
A fun colloquium today
Physics of NASCAR at 4pm in SH101
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

3. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

4. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Problems 10 – 14]
A car starts from rest and accelerates with the acceleration for 15 s. It then travels at a constant speed for 50s and then slows down at the rate of 1 m/s2 to a full stop. Ignore road friction and air resistance. Be sure to express vector quantities in vector form.
10. What is the velocity after 40s from the start?
Solution: Since the acceleration is applied only in x-direction for the first 15s, the velocity at 15 seconds is the same as that at 40s from the start. Using the kinematic equation, we obtain
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

5. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Problems 10 – 14]
A car starts from rest and accelerates with the acceleration for 15 s. It then travels at a constant speed for 50s and then slows down at the rate of 1 m/s2 to a full stop. Ignore road friction and air resistance. Be sure to express vector quantities in vector form.
11. What is the total displacement after 50s from the start?
Solution: We divide the motion in two segments: one from 0 – 15s where the car was accelerated and 15 – 50, the 35 s period the car was running at the constant velocity of . Using the kinematic equation, we obtain
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

6. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Problems 10 – 14]
A car starts from rest and accelerates with the acceleration for 15 s. It then travels at a constant speed for 50s and then slows down at the rate of 1 m/s2 to a full stop. Ignore road friction and air resistance. Be sure to express vector quantities in vector form.
12. What is the velocity after 100s from the start??
Solution: Since the acceleration in opposite direction was applied on the car from 65s after the start of the motion at which time the car was running at a constant velocity, Using kinematic equation, we obtain
What is the acceleration?
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

7. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Problems 18 – 21]
A projectile is launched with a speed of 100 m/s at an angle of 30 o with respect to the horizontal axis. The magnitude of the gravitational acceleration is 9.8m/s2. Ignore the air resistance.
18.What is the projectile’s maximum altitude?
Solution: From the information given in the problem, we obtain
Initial speed
Launch Angle
X-component of the initial velocity
y-component of the initial velocity
Maximum altitude is determined only by the y component of the displacement!
In y
Solve for y
Since at the maximum altitude is y component of the velocity is 0
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

8. Problems 18 – 21]
A projectile is launched with a speed of 100 m/s at an angle of 30 o with respect to the horizontal axis. The magnitude of the gravitational acceleration is 9.8m/s2. Ignore the air resistance.
19.What is the distance of the projectile from the starting point when it lands?
Solution: Since the time the projectile takes to reach back to land is twice the time it takes to reach the maximum altitude.
Which component determines the flight time?
y-component
Time to reach
max altitude
Solve for t
Flight time is twice the time to reach the max height!!
The range is then determined only by the x component of the displacement!
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

9. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Problems 18 – 21]
A projectile is launched with a speed of 100 m/s at an angle of 30 o with respect to the horizontal axis. The magnitude of the gravitational acceleration is 9.8m/s2. Ignore the air resistance.
20.What is the projectile’s speed when it reaches the maximum altitude?
Solution: Since when the projectile reaches the maximum height the y component of the velocity becomes 0, the speed at the maximum altitude is the same is the magnitude of the x component of the initial velocity.
Initial speed
Launch Angle
X-component of the initial velocity
y-component of the initial velocity
x-component of
the final velocity
y-component of
the final velocity
Same as the
x-component
of the final velocity
So the final
speed is
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

10. Applications of Newton’s Laws
Suppose you are pulling a box on frictionless ice, using a rope.
What are the forces being exerted on the box? M T
Gravitational force: Fg
n= -Fg
Normal force: n
Free-body diagram T
Tension force: T
Fg=Mg
Total force: F=Fg+n+T=T
n= -Fg
Fg=Mg T
If T is a constant force, ax, is constant
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu
What happened to the motion in y-direction?

11. Example for Using Newton’s Laws
A traffic light weighing 125 N hangs from a cable tied to two other cables fastened to a support. The upper cables make angles of 37.0o and 53.0o with the horizontal. Find the tension in the three cables. x y
53o
37o T1
37o T2
53o
Free-body
Diagram T3
Newton’s 2nd law
x-comp. of net force
y-comp. of net force
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu

12. PHYS 1441-002, Spring 2009 Dr. Jaehoon Yu
Example w/o Friction
A crate of mass M is placed on a frictionless inclined plane of angle q.
Determine the acceleration of the crate after it is released. x y x y x y n M a n Fg d
Free-body
Diagram
F= -Mg q q
Supposed the crate was released at the top of the incline, and the length of the incline is d. How long does it take for the crate to reach the bottom and what is its speed at the bottom?
Wednesday, Mar. 4, 2009
PHYS , Spring 2009 Dr. Jaehoon Yu